As an automobile exhaust gas purification catalyst, a three-way catalyst simultaneously oxidizing the CO and HC in exhaust gas and reducing NOx at a stoichiometric air-fuel ratio is being used for purification. As a three-way catalyst, for example, a heat resistant base member comprised of cordierite etc. on the surface of which a coat layer made of γ alumina is formed and supporting on the coat layer platinum Pt, rhodium Rh, palladium Pd, or another precious metal is widely known.
A three-way catalyst for exhaust gas purification of a gasoline engine, in particular a start catalyst arranged near the engine, has to be improved in low temperature activity the most among three-way catalysts since the HC and NOx exhausted at the time of cold start account for about 80% of the emissions.
However, up until now, no specific means for raising the low temperature activity have been known, so the practice had been to increase the supported amount or strengthen the insulation of the exhaust pipe to meet emission controls.
Here, among the HC and NOx accounting for the majority of the exhaust at the time of cold start, Rh has a high purification performance of NOx. Further, the purification of HC has an integral relationship with the purification of NOx in a three-way catalyst reaction mechanism.
Therefore, by using Rh, improvement of the low temperature activity may be expected.
However, in the case of Rh, it is not possible to use alumina (Al2O3) as a support like with Pt or Pd. This is because, as known in the past from C. Wong and R. W. McCabe, Journal of Catalysis, 119, 47-64 (1989) etc., in Rh/γ alumina systems etc., the Rh becomes solid solute in the alumina support in an oxidizing atmosphere and conversely precipitates in a reducing atmosphere, so cannot be stably maintained as a catalyst. This is because rhodium oxide (Rh2O3) has a corundum type crystal structure the same as α alumina, so easily becomes solid solute in alumina.
For this reason, as disclosed in Japanese Patent Publication (A) No. 4-219140 etc., zirconia (ZrO2), which differs from Rh2O3 in crystal structure and will not become solid solute, has been used as a support to improve the low temperature activity and improve the durability.
However, Rh is extremely expensive, so for broader application, there was the problem that it was necessary to greatly reduce the amount of catalyst metal.
Further, Rh will not become solid solute in zirconia, but the zirconia itself easily sinters at the usage temperature of the catalyst, so in the end, degradation as a catalyst was also unavoidable.
Zirconia easily sinters because supporting Rh in a high temperature sintered zirconia is difficult, so to secure the supporting property of Rh, it was necessary to use low temperature sintered zirconia. For example, Japanese Patent Publication (A) No. 2002-282692 shows a method of adding La etc. to ZrO2 and supporting Rh on a support sintered at 500° C. by the ion adsorption method.
However, the exhaust temperature rises to 800° C. or more during automobile operation. According to experiments by the inventors, a zirconia support sintered at about 500° C. had an initial surface area of 100 m2/g, but fell to 40 m2/g when the highest temperature reached 800° C. or to 30 m2/g when the highest temperature reached 900° C. The zirconia particle size (average size) became coarser compared with the initial state 8 nm—reaching 25 nm after reaching 800° C. and 33 nm after reaching 900° C. Therefore, the problem of the sintering of the zirconia as an Rh support is derived from the fact that it is difficult to support Rh on high temperature sintered zirconia.
Further, a catalyst supporting Rh as ions by adsorption had the problem of deterioration of the activity compared with a catalyst supporting the same as particles.
To deal with this, various methods for supporting precious metals as colloids with higher activity than ions have been proposed in the past.
Japanese Patent Publication (A) No. 2000-279818 and Japanese Patent Publication (A) No. 2000-279824 disclose methods of supporting previous metals as polymer chelate metal colloids. While an adsorption method, tens of hours are required for supporting them. Further, with the sole practical PVP colloid, the supporting efficiency was a low one of about 30% and the improvement in performance was small.
Japanese Patent Publication (A) No. 2005-296733 discloses the method of using a metal colloid comprised of a protective agent and a catalyst metal to support a precious metal on a support by evaporation to dryness. However, with this method, basically the supporting density falls. Further, evaporation to dryness is used to secure the supporting ability. In the final analysis, there was no great difference from the methods of Japanese Patent Publication (A) No. 2000-279818 and Japanese Patent Publication (A) No. 2000-279824.
Further, a catalyst supporting a precious metal as a polymer stabilized metal colloid like the above is better in initial performance and more advantageous in durability in some respects as well compared with a catalyst supporting a precious metal by the impregnation method used in the past, but in the final analysis, there was the defect that under tough durability test conditions, the performance dropped compared with a catalyst supported by impregnation.
That is, a polymer stabilized colloid is dispersed as a colloid by the hydrophilic groups of the polymer. While the individual colloid particles may be small, a large number of colloid particles agglomerate to form large secondary particles. If evaporating these to dryness, they are supported as coarse secondary particles. Under a high temperature durability test, large sintering occurs. Compared with the time of impregnation, while the initial performance may be good, under a high temperature durability test, the performance drops and the superiority is substantially lost.
Furthermore, Japanese Patent Publication (A) No. 2004-82000 discloses a method of supporting a composite metal colloid having a center part of a precious metal and a surface part of a transition metal other than a precious metal by evaporation to dryness. In this method, the stability of a colloid such as a polymer stabilized colloid cannot be obtained. Further, problems similar to Japanese Patent Publication (A) No. 2000-279818, Japanese Patent Publication (A) No. 2000-279824, and Japanese Patent Publication (A) No. 2005-296733 using evaporation to dryness cannot be avoided.
Japanese Patent Publication (A) No. 2005-279435discloses a method of adding a basic salt of La etc. to an acidic Rh+ZrO2 solution, reducing and making the Rh precipitate and evaporating to dryness the result to support the precious metal, and suppressing sintering by the crystal lattice of Rh matching the crystal lattice of the support. Even with this method, problems similar to Japanese Patent Publication (A) No. 2000-279818, Japanese Patent Publication (A) No. 2000-279824, and Japanese Patent Publication (A) No. 2005-296733 using evaporation to dryness cannot be avoided.
In this way, up to now, there was the problem that the method of supporting a precious metal as colloid particles was low in supporting efficiency.